Prune tree fruit set is tricky business! Good fruit set occurs under a relatively narrow band of conditions and then requires thinning if good fruit size will be achieved. Franz Niederholzer (UCCE Colusa & Sutter-Yuba Counties) and I discuss what UC research has learned about bloom conditions and what growers can do to manage fruit set.
Thinning following good bloom conditions (65-80 F). Here is the thinning calculator: http://www.sacvalleyorchards.com/prunes/horticulture-prunes/prune-thinning-calculator/
Reference date (the timing for thinning) is typically late April or early May, approximately one week after the pit tip begins to harden. Specifically, the reference date is when 80-90% of the fruit have a visible endosperm (see a photo of what that looks like at: http://www.sacvalleyorchards.com/prunes/horticulture-prunes/thinning-prunes/)
Options during poor bloom conditions: http://www.sacvalleyorchards.com/prunes/bloom/bad-bloom-weather-options/
Thanks to the Pistachio Board of California and Almond Board of California for their generous financial support. Music by Muriel Gordon.